1. Field of the Invention
The present invention relates to an optical device for the correction of aberrations affecting an image. In particular, a device according to the invention makes it possible to correct the distortion due to a spherical concave mirror that is inclined with respect to the direction at which this mirror is observed.
The invention can be applied especially but not exclusively to a helmet viewfinder for the pilot of a fighter aircraft or helicopter or for the operator of a training simulator.
2. Discussion of the Background
A helmet viewfinder is an image-presenting device that is integrated into a helmet. The viewfinder enables the wearer of the helmet, for example the pilot of an aircraft in flight, to observe visual information simultaneously with the view of the landscape or of the pilot's cabin, which he perceives most usually through a protective visor.
The presentation of appropriate information, for example in the form of symbols, provides piloting and navigation assistance. Thus, for armed vehicles, the presentation of a reticule provides assistance in the aiming of a weapon.
The information may also consist of an image of the landscape acquired by sensors other than the eye of the helmet wearer such as infrared image sensors or light intensifiers to complement or replace direct viewing.
Inside the helmet, an image generator comprises an imager whose screen, for example a cathode-ray tube screen or a liquid crystal screen, enables an image to be displayed.
The image is most usually conveyed by a relaying optic up to a combiner which presents the conveyed image in a state where it is superimposed on the view of the landscape.
In order that the pilot may simultaneously observe the landscape viewed directly at infinity and the image from the imager, the latter image is also focused at infinity by a collimation optic.
When the combiner is formed by a simple semi-reflective flat plate, the collimation of the image can be achieved by an optic placed between the imager and the combiner. A prior art embodiment such as this has the major drawback of requiring a collimation optic that requires far too much space in relation to the restricted field of view that is obtained.
To reduce the space requirement, a combiner with optical power has been proposed. A combiner such as this provides its user with both the collimation of the image and the superimposition of the collimated image with the view of the landscape.
The prior art has a very extensive variety of devices comprising a combiner with optical power. Of more particular interest are image-presenting systems that comprise a spherical concave mirror to collimate the image.
A concave spherical mirror achieves an average quality collimation of an image placed at a particular point in space located on the axis of the mirror and at a distance from this mirror equal to half its radius of curvature. By placing an imager at this point, the eye located on the axis of the mirror receives rays coming from the imager after they are reflected on the spherical mirror. These rays are parallel and lead to the perception, by the eye, of a collimated image. If, furthermore, the mirror is semi-reflective, it enables the same eye to observe the landscape by transparency. However, in a device such as this, the imager would have to lie on the axis of the semi-transparent spherical mirror and it would conceal the user's field of view.
To clear the user's view, the spherical mirror is inclined with respect to the normal to his/her face and the user's eye is no longer on the axis of the mirror. This arrangement has the drawback of resulting in a collimated image that is affected by optical aberrations, especially off-centring aberrations, which need to be corrected, at least partially.
The inclination of the spherical concave mirror afflicts the collimated image with distortion, known as off-centring distortion of the second kind, characterized by a convergence of the verticals and an apparent curvature of the horizontals.
In a patent filed under No. 97 09893 on Aug. 1, 1997 by the present Applicant, an aspherical mirror with an adapted shape enables a correction of the off-centring distortion of the second kind.
The particular surface of the proposed aspherical mirror enables a modification of the light rays in order to rectify the effects of the spherical concave mirror on the horizontals and verticals of the image observed and thus ensure a correction of the distortion. This correction is achieved by the introduction through the aspherical mirror, of an off-centring distortion of the second kind to compensate for the distortion of the same type due to the spherical concave collimation mirror used off-axis. The aspherical mirror has the effect of making the verticals parallel and the horizontals rectilinear in the collimated image. The image is rectified and orthoscopic but the overall shape of the mirror causes a local amplification of the aberrations, and especially of astigmatism. The correction of the distortion enabled by this invention is limited by a deterioration of the resolution of the image.